Method for recovering precious metal from metal carrier catalyst

ABSTRACT

A method for recovering precious metal held in a metal carrier catalyst from said catalyst, that includes a step (1) in which the catalyst is heated in a nitric acid solution, and a step (2) in which the post-step (1) catalyst is heat treated in a sulfuric acid solution.

TECHNICAL FIELD

The present invention relates to a method for recovering a catalyst precious metal from a metal carrier catalyst used for exhaust gas purification.

BACKGROUND ART

In exhaust gas purification catalysts installed in numerous two-wheeled vehicles and some four-wheeled vehicles, a catalyst coating layer containing a precious metal is formed on a metal carrier base material made of a material such as heat-resistant stainless steel. In the case of quantifying the precious metal loaded on a metal-supported catalyst by a third party, it is necessary to recover the precious metal in the catalyst coating layer by removing the catalyst coating layer with acid and the like.

However, many conventional precious metal recovery methods have a complicated process. An excessively complicated recovery method results in an increase in human error, and the amount of precious metal eliminated from the recovery system increases, as the number of steps becomes excessively large. Consequently, there was a difference between the actual amount of precious metal loaded on a catalyst and the amount of precious metal recovered in the case of conventional recovery methods.

For example, the methods used to recover precious metal (such as platinum, palladium or rhodium) can be broadly divided into three dissolution steps consisting of acid dissolution (hydrochloric acid), alkaline dissolution (sodium peroxide) and acid dissolution (concentrated hydrochloric acid).

More specifically, in these methods, precious metal is recovered by going through the steps indicated below.

1. Acid Dissolution (Hydrochloric Acid)

A sample (having a catalyst coating layer containing a precious metal formed on a metal honeycomb body) is placed in a 2000 ml beaker followed by the addition of hydrochloric acid at a volume ratio of 1:1 (volume hydrochloric acid:volume of water=1:1) at normal temperature to the metal honeycomb body to fragmentize the honeycomb body. The hydrochloric acid is added several times so as to adequately fragmentize the honeycomb body. The outer cylinder that remains following dissolution is removed followed by washing with an ultrasonic cleaner. The solution resulting from washing and the original solution are mixed and designated as Solution A (the solution may be concentrated to 1000 ml by heating in the case the volume thereof is excessively large). Solution A is then filtered with a filtering device followed by washing the insoluble matter three times with distilled water. The filtrate and the washing solution are then combined and designated as Solution B.

2. Alkaline Dissolution (Sodium Peroxide)

A high alumina crucible is weighed to a constant weight and the resulting weight is designed as W1. The filter paper is placed in the high alumina crucible and the high alumina crucible is placed in a dryer and dried at 120° C. After drying, the high alumina crucible is placed in a muffle furnace and the filter paper is carbonized for 30 minutes at 300° C. followed by raising the temperature to 700° C. and holding at that temperature for 1 hour and then cooling. The powder and high alumina crucible following combustion are then weighed and the resulting weight is designated as W2. An amount of sodium peroxide equal to four times the weight of the powder (W2-W1) is placed in the high alumina crucible and uniformly mixed. A single layer of sodium peroxide is then coated on the surface thereof. The high aluminum crucible is placed in a muffle furnace followed by raising the temperature to 770° C. and holding at that temperature for 0.5 hours. Subsequently, the power of the muffle furnace is turned off and the high alumina crucible is removed from the furnace after it has cooled. The high alumina crucible is placed in a 1000 ml beaker followed by the addition of 100 ml of water and dissolving by heating by placing on a heating plate.

3. Acid Dissolution (Concentrated Hydrochloric Acid)

The high alumina crucible is removed from the heating plate and allowed to cool followed by slowly adding concentrated hydrochloric acid in an amount equal to 100 times the amount of sodium peroxide, again heating the high alumina crucible on the heating plate to boiling, removing the high alumina crucible from the beaker and rinsing the high alumina crucible with distilled water. The resulting solution is designed as Solution C. Solution C and Solution B are then combined followed by the addition of a suitable amount of concentrated hydrochloric acid to adjust the acidity of the solution to 1.5 M to 3.0 M. The solution is brought to a constant volume in a bottle having a volume of 1000 ml or 2500 ml, and the resulting volume is designated as Total Volume V.

Although the aforementioned method is used to measure the amount of precious metal loaded on an exhaust gas purification catalyst installed in a two-wheel vehicle by the China Vehicle Technology Service Center (CVTSC), in addition to the process being complex and time-consuming, the amount of precious metal recovered tends to indicate values that are lower than the actual amount of precious metal loaded on the catalyst.

PRIOR ART DOCUMENTS Non-Patent Documents

Non-Patent Document 1: China Vehicle Technology Service Center web site: http://www.cvtsc.org.cn/cvtsc/zcfg/693.html, accessed: Feb. 10, 2012

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

Thus, an object of the present invention is to provide a method for efficiently recovering precious metal in fewer steps than in the prior art.

Means for Solving the Problems

As a result of conducting extensive studies on the aforementioned problems, the inventor of the present invention found that precious metal of the catalyst is substantially recovered using a method that only goes through two steps consisting of treatment of a metal carrier catalyst with nitric acid and treatment with sulfuric acid, thereby leading to completion of the present invention.

Namely, the present invention includes the inventions indicated below.

[1] A method for recovering precious metal loaded on a metal carrier catalyst from the catalyst, comprising:

1) a step for heating the catalyst in a nitric acid solution, and

2) a step for heat-treating the catalyst obtained by step 1) in a sulfuric acid solution.

[2] The method of [1], wherein the heating of step 1) is carried out at 90° C. or higher for about 40 minutes to 60 minutes.

[3] The method of [1] or [2], wherein the heating of step 2) is carried out at 90° C. or higher for about 30 minutes to 120 minutes.

[4] The method of any of [1] to [3], further comprising a step for washing the catalyst obtained by step 1) with water.

Effects of the Invention

In the past, it was thought to be necessary to go through a complicated process in order to measure the amount of precious metal loaded on a metal carrier catalyst. However, as shown in the flow chart of FIG. 1, since the recovery method according to the present invention is simplified in comparison with conventional methods, the amount of precious metal removed from the recovery system during the course of going through a large number of steps can be reduced. In addition, despite being simplified in comparison with conventional methods, the method of the present invention enables any of a plurality of different precious metals to be substantially completely recovered. Moreover, since it is necessary to concentrate the recovered precious metal solution during quantification thereof, if the amount of solution containing precious metal is reduced accompanying the reduction in the number of steps used to dissolve the precious metal from the metal carrier catalyst, both recovery time as well as concentration time can be shortened.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a flow chart from recovery of precious metal to quantification of the recovered precious metal according to the present invention.

FIG. 1B is a flow chart from recovery of precious metal to quantification of recovered precious metal according to the present invention.

FIG. 2 is a graph indicating the results of examining the concentration of nitric acid that does not have an effect on changes in base material weight.

FIG. 3 is a graph indicating the results of examining the boiling time that does not have an effect on changes in base material weight.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention relates to a method for recovering precious metal loaded on a metal carrier catalyst from the catalyst, comprising: 1) a step for heating the catalyst in a nitric acid solution, and 2) a step for heat-treating the catalyst obtained by step 1) in a sulfuric acid solution.

In the case of using in the present description, a “metal carrier catalyst” refers to an exhaust gas purification catalyst composed of a metal carrier base material and a catalyst coating layer formed on the base material that contains a porous oxide carrier and one or a plurality of catalyst precious metals. Examples of materials used for the carrier base material include, but are not limited to, stainless steel and heat-resistant steel. In addition, there are no particular limitations on the form of the carrier base material and may be a metal honeycomb structure or a metal honeycomb structure having punched holes formed therein.

Any arbitrary material can be used for the porous oxide carrier and catalyst precious metals that compose the catalyst coating layer. Examples of porous oxides that can be used include γ- or θ-alumina, titania, zirconia, ceria, silica and compound oxides composed of a plurality of types selected therefrom. Although the precious metal recovery method according to the present invention is able to efficiently recover any precious metal, it enables the recovery of platinum, palladium, rhodium, iridium and ruthenium particularly preferably.

In the first step, a metal carrier catalyst is immersed in a nitric acid solution. Not to be bound by theory, it is thought that the nitric acid causes the formation of an oxide film on the metal carrier base material, thereby fulfilling the role of preventing the sulfuric acid used in the subsequent step from dissolving the base material as a result of this oxide film serving as a protective coating. The nitric acid solution used in the present invention refers to that which has been prepared as an aqueous solution at a concentration that does not aggressively dissolve the base material. Since the protective effect of the nitric acid on the metal carrier base material converges to a prescribed degree even in the case the nitric acid concentration has become excessively high, a person with ordinary skill in the art is able to reasonably select a nitric acid concentration that allows the obtaining of the desired protective effect, and the concentration of nitric acid in the aqueous solution is, for example, 1% by weight to 20% by weight, preferably 5% by weight to 15% by weight, and more preferably 8% by weight to 13% by weight. The concentration of the nitric acid may be varied corresponding to the properties of the catalyst coating layer and the type of precious metal recovered, and may also be in the form of a mixed solution by adding other acid to the nitric acid solution.

The metal carrier catalyst immersed in the nitric acid solution is heated for a prescribed amount of time at a temperature of 90° C. or higher, such as a temperature of 100° C. to 120° C., under atmospheric pressure. Heating time is suitably determined. This is to enable the protective effect of the nitric acid on the metal carrier base material to converge to a prescribed degree even in the case of heating for an excessively long time. Accordingly, a person with ordinary skill in the art is able to reasonably select a heating time that allows the obtaining of the desired protective effect. For example, although varying according to temperature conditions, heating in the nitric acid solution is carried out for 15 minutes to 180 minutes, preferably 30 minutes to 120 minutes and more preferably 30 minutes to 60 minutes.

Following heating, the precious metal carrier catalyst is removed from the nitric acid solution and immersed in a sulfuric acid solution in order to remove the catalyst coating layer. The sulfuric acid solution used in the present invention refers to that which has been prepared as an aqueous solution at a concentration that is suitable for removing the catalyst coating layer. For example, the concentration of sulfuric acid in the sulfuric acid aqueous solution is 2.5% by weight to 60% by weight, preferably 5% by weight to 50% by weight and more preferably 8% by weight to 30% by weight. Although the sulfuric acid concentration is preferably not excessively high since it erodes the outer cylinder, the concentration of sulfuric acid may be suitably varied corresponding to the properties of the catalyst coating layer and type of precious metal recovered, and may also be in the form of a mixed solution by adding other acid to the sulfuric acid solution.

The metal carrier catalyst immersed in the sulfuric acid solution is heated for a prescribed amount of time at a temperature of 90° C. or higher, such as a temperature of 100° C. to 120° C., under atmospheric pressure. Heating time can be suitably determined, and is, for example, 5 minutes to 240 minutes, preferably 10 minutes to 180 minutes, and more preferably 30 minutes to 120 minutes.

The metal carrier catalyst treated with acid as described above may be washed once or a plurality of times by immersing in water. During this washing, shaking may be employed in order to elute as much catalyst metal into the water as possible. Elution efficiency is enhanced by carrying out ultrasonic treatment instead of or simultaneous to shaking. In addition, the metal carrier catalyst is not required to be completely immersed, but rather a portion thereof may be exposed above the water surface or the metal carrier catalyst may be inverted during the course of washing.

Prior to submitting for quantitative analysis, the solution obtained from the aforementioned acid treatment and washing is concentrated by heating and then denitrated. Continuing, although the precious metal to be recovered is separated from other metals, the separation method can be suitably determined corresponding to the type of precious metal, and for example, tellurium co-precipitation is preferable in the case of separating a precious metal such as platinum, palladium or rhodium by co-precipitation.

Quantification of precious metal can be carried out using a spectroscopic analysis method commonly known among persons with ordinary skill in the art, examples of which include high-frequency inductively coupled plasma atomic emission spectrometry (ICP-AES), electrothermal atomic absorption spectrometry, high-resolution ICP mass spectrometry (ICP-MS), absorption spectrophotometry and non-dispersive infrared absorption. Methods enabling simultaneous multi-element analyses and performative analyses such as ICP analyses are preferable. In the case of measuring the amount of precious metal by tellurium co-precipitation, although precious metal can be quantified by ICP-MS, this is easily affected by contaminants due to the excessively high sensitivity thereof, and the high sample dilution rate results in increased susceptibility to increases in error. On the other hand, ICP-AES does not have such shortcomings while also having a wider linear range in the calibration curve, thereby making this preferable.

In the method according to the invention of the present application, since a metal carrier catalyst is heated in two types of acids as previously described, the catalyst coating layer is easily removed in a short period of time. In addition to it being easy to remove the catalyst coating layer, the metal honeycomb body and outer cylinder that covers the honeycomb body are not damaged. On the other hand, conventional methods require a long time for treatment since acid treatment is carried out at normal temperature. Moreover, conventional methods are more complex in comparison with the invention of the present application since the invention of the present application does not require three steps consisting of a step for filtering a precious metal solution obtained following acid treatment, a step for treating with sodium peroxide, and a step for treating with concentrated hydrochloric acid.

The following provides a more detailed explanation of the present invention using the examples indicated below. Furthermore, the present invention is not limited to these examples.

EXAMPLES Examination of Nitric Acid Concentration

Nitric acid solutions were prepared having concentrations over the range of 0.5% by weight, 1% by weight, 5% by weight, 10% by weight, 15% by weight and 20% by weight. A metal carrier was immersed in each nitric acid solution followed by boiling for 30 minutes. The reduction rate of the base material was calculated in comparison with the base material weight prior to nitric acid treatment in accordance with the equation indicated below. The results are shown in FIG. 2.

Base material reduction rate (%)=(base material weight before treatment−base material weight after treatment)÷base material weight before treatment×100

Examination of Nitric Acid Boiling Time

A nitric acid solution having a concentration of 9% by weight was prepared and a metal carrier was immersed in the nitric acid solution followed by boiling for 5 minutes, 10 minutes, 20 minutes, 30 minutes or 40 minutes. The base material reduction rate was calculated in comparison with the base material weight prior to nitric acid treatment. The results are shown in FIG. 3.

Examples

Commercially available metal carrier catalysts were prepared on which platinum, palladium and/or rhodium were respectively loaded in prescribed amounts followed by boiling for 40 minutes or 60 minutes in a nitric acid solution having a concentration of 9% by weight or 10% by weight. The metal carrier catalyst was recovered from the acid solution and the nitric acid solution following recovery was designated as Solution A. Solution A was then concentrated to a suitable volume by heating. Continuing, the metal carrier catalyst was boiled for 20 minutes to 120 minutes in a sulfuric acid solution having a concentration of 8% by weight to 30% by weight. The catalyst was recovered from the sulfuric acid solution, the sulfuric acid solution following recovery was designated as Solution B, and Solution B was added to Solution A. The metal carrier catalyst following sulfuric acid treatment was washed with pure water. Subsequently, the metal carrier catalyst was then washed by placing in an ultrasonic cleaner. These washing solutions were added to the aforementioned mixed solution of Solution A and Solution B followed by concentrating by heating. Although the amount of the aforementioned acid solution prior to concentration by heating varies corresponding to the volume of the metal carrier catalyst used, the amount of the solution following concentration was adjusted to within the range of about 500 ml to 2000 ml.

After transferring a suitable amount of the solution following concentration by heating to a beaker, the solution was subjected to denitrification. A tellurium solution and a stannous chloride solution were then added followed by carrying out tellurium co-precipitation treatment. Continuing, the co-precipitate was removed by suction filtration, and after dissolving with acid and adjusting to a prescribed volume with hydrochloric acid, the resulting solution was subjected to ICP-AES (Model CIROS-120, Rigaku Corp.) and the amount of each precious metal was measured at the wavelengths indicated below (platinum: 214.423 nm, palladium: 248.892 nm, rhodium: 343.892 nm). Measurement results were calculated in the manner indicated below based on the percentage (%) of each precious metal recovered.

Recovery rate=(quantitatively determined amount of precious metal÷amount of precious metal actually loaded)×100  [Equation 1]

The concentrations of acids used and boiling times are indicated in the following table in addition to the measurement results.

TABLE 1 Nominal Analysis Ex. Boiling Results (%) No. Acid Used Weight (%) Time (min) Pt Pd Rh Ex. 1 Nitric acid 10 40 95.3 98.9 95.7 Sulfuric acid 8 30 Ex. 2 Nitric acid 10 40 94.5 97.4 95.3 Sulfuric acid 10 30 Ex. 3 Nitric acid 9 60 96.8 98.7 99.1 Sulfuric acid 10 60 Ex. 4 Nitric acid 9 60 97.3 96.9 98.7 Sulfuric acid 10 60 Ex. 5 Nitric acid 9 40 96.9 99.1 97.4 Sulfuric acid 10 40 Ex. 6 Nitric acid 9 40 97.6 96.9 97.7 Sulfuric acid 20 20 Ex. 7 Nitric acid 9 40 99.3 98.7 99.1 Sulfuric acid 30 20 Ex. 8 Nitric acid 9 40 95.0 96.3 98.5 Sulfuric acid 10 40 Ex. 9 Nitric acid 9 40 97.2 98.1 99.1 Sulfuric acid 10 120 Ex. 10 Nitric acid 9 40 94.6 95.2 99.4 Sulfuric acid 15 60 Ex. 11 Nitric acid 8 40 99.8 97.3 100 Sulfuric acid 40 90

As is clear from the above table, the recovery rate of precious metal according to the method of the present invention demonstrated high efficiency of about 95% or higher regardless of the type of precious metal.

Comparative Example

Metal carrier catalysts respectively loaded with prescribed amounts of platinum, palladium and rhodium were prepared, the metal carrier catalysts were placed in a 2000 ml beaker followed by the addition of hydrochloric acid at a volume ratio of 1:1 at normal temperature by dividing into several additions until the inner core had sufficiently fragmentized. After dissolving at room temperature, the remaining outer cylinder was removed followed by washing by placing in an ultrasonic cleaner. The solution obtained after washing and the original solution were then combined.

The aforementioned solution was filtered with a filtration device and insoluble matter was washed three times with distilled water. The filtrate and washing solution were then combined.

A high alumina crucible was weighed to a constant weight and the resulting weight was designated as W1. The filter paper was placed in the high alumina crucible followed by placing the high alumina crucible in a dryer and drying at 120° C. After drying, the high alumina crucible was placed in a muffle furnace followed by carbonizing the filter paper for 30 minutes at 300° C., raising the temperature to 700° C., holding at that temperature for 1 hour and cooling.

The powder and high alumina crucible were weighed following combustion and the resulting weight was designated as W2. An amount of sodium peroxide equal to four times the weight of the powder (W2-W1) was placed in the high alumina crucible and uniformly mixed. A single layer of sodium peroxide was then coated on the surface thereof. The high aluminum crucible was placed in a muffle furnace followed by raising the temperature to 770° C. and holding at that temperature for 0.5 hours. Subsequently, the power of the muffle furnace was turned off and the high alumina crucible was removed from the furnace after it had cooled.

The high alumina crucible was placed in a 1000 ml beaker followed by the addition of 100 ml of water and dissolving by heating by placing on a heating plate. The high alumina crucible was then removed from the heating plate and allowed to cool followed by slowly adding concentrated hydrochloric acid equal to 10 times the amount of sodium peroxide, again heating on the heating plate to boiling, removing the high alumina crucible from the beaker, and rinsing the high alumina crucible with distilled water.

The resulting precious metal solutions were then combined followed by the addition of a suitable amount of concentrated hydrochloric acid to adjust the acidity of the solution to 1.5 M to 3.0 M (liquid volume: about 2000 ml). Subsequently, tellurium co-precipitation was carried out in the same manner as in the examples followed by ICP-AES. The results are shown in the table below.

TABLE 2 Pt Pd Rh 214.423 nm 248.892 nm 343.489 nm 40 ml aliquot (1) 87.3 91.3 91.5 after adjusting (2) 87.2 89.4 91.7 to 2000 ml (n = 2) Average 87.3 89.9 89.5

As is indicated in the table above, the amounts of precious metals recovered varied from the actual loaded amounts in the case of comparing with the method of the present invention, and the recovery rate for platinum was particularly low. 

1. A method for recovering precious metal loaded on a metal carrier catalyst from the catalyst, comprising: 1) a step for heating the catalyst in a nitric acid solution, and 2) a step for heat-treating the catalyst obtained by step 1) in a sulfuric acid solution.
 2. The method according to claim 1, wherein the heating of step 1) is carried out at 90° C. or higher for about 40 minutes to 60 minutes.
 3. The method according to claim 1 or 2, wherein the heating of step 2) is carried out at 90° C. or higher for about 30 minutes to 120 minutes.
 4. The method according to any of claims 1 to 3, further comprising a step for washing the catalyst obtained by step 2) with water. 